1. Technical Field
The present invention relates to a deformable mirror appropriate for use in retinal cameras, heads-up displays, astronomical telescopes, laser irradiation devices and so on. The present invention is also relates to a device for observing the retina of an eye that casts light beam from a photographing light source to an eye to be examined and records the image of the light beam reflected from the retina of the eye as a retinal image, for diagnosing the retina of the eye.
2. Related Art
However, since a human eye optical system is not ideal as a photographing object, sufficient resolution cannot be necessarily achieved. Therefore, in order to compensate for the deformation of the wavefront of a human eye optical system, deformable mirrors using a piezoelectric effect have been in use.
The device such as a camera for observing the retina of an eye is used by ophthalmologists and ophthalmic opticians to photograph the image of the retina of an eye for inspecting the state of the retina, hemorrhage on the retina of the eye, and so on. Incidentally, the human eyes optical system is composed of the cornea, the lens, the vitreous body and others with, unlike an ideal optical system used as a basis of the geometrical optics, some deformation. In particular in the clinical field of ophthalmology, the image of the retina of the eye is required to be clear and of little aberration because the extent of difference of the examined eye from a normal eye is used as diagnosis information. However, because the optical system for the human eyes constituting the photographing device is not ideal, in some cases sufficient resolution cannot be achieved. Therefore, to compensate for the deformation of the wavefront of the optical system for the human eyes, the deformable mirrors using the piezoelectric effect have been in use.
However, the conventional deformable mirrors using the piezoelectric element require a high voltage applied to the piezoelectric element and needs to use, as an electronic control circuit, a piezoelectric element with a high dielectric strength that is expensive. Therefore, commercially available retinal cameras employ deformable mirrors using electrostatic attraction that can be actuated with a lower drive voltage in comparison with the piezoelectric type.
In the case of a deformable mirror using electrostatic attraction, however, electrodes are arranged at a high density so that the deformation of the mirror can be controlled minutely. To prevent a creeping discharge and a dielectric breakdown between the electrodes and the wires, there should be proper intervals between them. For example, to prevent a creeping discharge, there should be intervals of at least 0.2 to 0.3 mm for a drive voltage of 300V.
Conventionally, a single-piece glass or ceramics substrate is used as an electrode substrate for a deformable mirror. In the conventional structure, the wiring of the electrode substrate is made by directing the wires from the front side to the back side of the electrode substrate by through-hole processing and connecting the wires to a connector cable through the two layers on the front and back sides. However, when a drive voltage of a few hundred volts is used, the degree of freedom in the layout of the wiring is too low with a two-layer structure to satisfy the necessity of providing sufficient intervals suitable for a high voltage between the wires and arranging the electrodes at a high density. On the other hand, when a ceramics substrate on which wiring can be formed in multiple layers is used as the electrode substrate, there are advantages that a desired flatness can be achieved by processing and that a material with an expansion coefficient close to that of the membrane can be selected, but there is also a disadvantage that the material and processing costs are high.
The present invention has been made to solve the above problems and it is, therefore, an object of the present invention to provide a deformable mirror in which the distance between an electrode substrate and a membrane can be accurately maintained and which can be produced at a low material cost.